The present invention relates generally to variable high-current resistors, and more particularly to a variable high-current resistor for use as a protective element in power switching applications, in which carbon is used as a resistance material. Moreover, the present invention relates to circuit arrangements making use of this high-current resistor. In the case of such circuit arrangements, a circuit-breaker, a power semiconductor, a contactor or a vacuum contactor can be preferably provided as the circuit element.
In electrical low-voltage distribution systems, circuit-breakers must be able to interrupt the expected maximum short-circuit current in the event of a short-circuit. In the proximity of bus bars, this current can amount to 50 kA or even 100 kA. Circuit-breakers having this interrupting capacity must open the contacts within a period of time that is short relative to the duration of a system power-supply period and build up a high arc voltage in order to adequately limit the short-circuit current.
The latter circuit-breakers are complex in design and, accordingly, expensive. To be able to use simpler circuit-breakers having a smaller breaking capacity in the case of large, prospective short-circuit currents, equipment or devices are required to aid the circuit-breaker in limiting and interrupting the current in the event of a short-circuit.
Known methods heretofore have used so-called current limiters, which are connected in series to the circuit-breaker, as shown, e.g., in FIG 1a. "Limiters" are generally understood to be switching devices that do not have a breaker mechanism, so that in the event of a short-circuit, the contacts are opened by the current forces: the resulting arc voltage helps the circuit-breaker to limit and interrupt the current flowing in the shorted circuit.
Another possibility for limiting current consists in connecting a resistor in series with the circuit-breaker, which resistor in the case of nominal current or permissible overload current has a very low value and becomes highly resistive when a certain current threshold is exceeded within a short time. Thus, an additional resistor is provided in the shorted circuit which limits the current to harmless values. Resistors having the above described characteristic are described as PTC resistors (Positive Temperature Coefficient) or thermistors. A circuit arrangement that makes use of a resistor of this type for low-voltage applications has been proposed by the WO-A-91/12643. The variable high-current resistor it describes consists essentially of a carbon-black-filled polyethylene layer with a PTC effect. The same had essentially already been proposed by the WO-A-90/00825 for protective devices used for low voltages.
Moreover, the EP-A-0 487 920 describes in detail a PTC element having at least one resistor core of an electrically conductive polymer composition with a positive temperature coefficient, in the case of which each resistor core has two, preferably parallel surfaces, at least one boundary layer being connected to electrodes to introduce the current into the PTC element. To guarantee the PTC effect in the case of this high-current resistor that is usable as a protective element, the polymer resistor core is joined by its base areas to an electrode, at least one surface forming a so-called free contact with the associated electrode, and one pressure device being present, which exerts a pressure in a perpendicular direction on the electrodes and the base areas of the resistor core of the conductive polymer layer. The contact is altered as the result of electrodynamic repulsion at high currents, a simultaneous increase in temperature supposedly forming a fluid phase in the polymer material.
In addition, the DE-A-0 272 807 discloses a regulating resistor, in the case of which changes in resistance are produced by changes in the pressure of an electromagnetic armature on a granular mass containing, in particular, carbon powder. Finally, a carbon pressure regulator is also known from the ETZ (A) 73, 199 (52), which in response to a pressure-dependent change in the contact resistance rests on columnar laminae or rings. This carbon pressure regulator is used to regulate a voltage or a current, in that, given a rising value of the controlled variable, an electromagnet is controlled to produce a rising magnetic force to counteract the prestressing of a column of carbon laminae. Supposedly, variations in resistance by the factor 300 are possible.
In contrast, the present invention is directed to the problem of developing a high-current resistor of the type mentioned at the outset, so that it can be used as a limiter with the level of security required of protection devices. The requisite circuit arrangements shall also be indicated.